The present invention relates to a process for reclaiming thermally strained polyester scrap material. The reclaimed material can be used, e.g, in the production of high-quality polyethylene terephthalate plastic articles. The process comprises shredding, purifying, and sorting the polyester scrap material during its reclaiming.
Today, it is no longer the sole object of the manufacturer to produce goods. To an increasing extent, the manufacturer is expected to provide for the disposal of its products after use as well. In connection with the reclaiming of plastic materials, an essential problem resides, because the plastics are partially degraded during their processing into commodity articles. Due to the fact that the used materials possess inferior optical and mechanical properties, it is, in general, not possible to use them again in the production of goods having the same good quality as the initial products.
German Offenlegungsschrift No. 34 33 791 discloses a process for producing plastic granules which are suitable for thermoplastic processing in an extruder, from scrap material obtained in the course of the extrusion of thermoplastic materials. The process comprises first cutting and optionally drying the material, then densifying the cut material, and finally comminuting the material into pellets which can be fed to the extruder. This known process is directed to the reclaiming of scrap material of a specific polymer, i.e., one whose mechanical, optical, and chemical properties are unchanged compared to the initial processing.
A process for the recovery of densified material from plastic films or plastic film scrap material is known from German Auslegeschrift No. 14 54 875. In this process, the films are first comminuted, then densified in an agglomeration apparatus, and transported by means of a blower equipped with a conveying line and an air separator, whereupon the densified material is comminuted in a cutting mill, to give pellets having approximately the size of the granules of the polymer raw material.
U.S. Pat. No. 3,344,091 is directed to a process for converting scrap polyester, wherein the scrap is comminuted to a certain particle size, mixed with an aromatic dicarboxylic acid ester and glycol, and heated to a temperature 25.degree. to 50.degree. C. above the boiling point of the glycol. After dissolution of the polyester and removal of the alcohol, a prepolycondensate having an average degree of polycondensation of less than 10 is obtained, which upon further heating is converted into a powder. This powder is subjected to a solid phase polycondensation process in an inert gas atmosphere. The process is based on a two-step procedure, the first step comprising the degradation of the scrap polymer into a low-molecular product, and the second step comprising the polycondensation of said low-molecular product to give a high-molecular product.
Like the process described above, this two-step process comprising a dissolution or degradation step and a condensation step is mainly directed to the post-condensation of scrap material of one single polymer having a specific degree of polycondensation. It is not intended to treat polymer scrap materials having different molecular weights in one process cycle.